1. Field of the Invention
The present invention relates to a switching power supply device used as a DC power source for electrical appliances.
2. Description of the Prior Art
Recently, electrical appliances such as a facsimile, a telephone set, a copying machine, other office automation equipment, home electrical appliances, or the like that require a supply of electricity during a standby period in addition to an operation period have been on the rise. Because these electrical appliances need a stable constant operating voltage, a switching power supply device capable of outputting a stabilized voltage has been used. Against the background of energy conservation in recent years, in addition to reducing the power consumed by the switching power supply device, it has become increasingly important to reduce the power consumed during the standby period that accounts for a larger proportion of time than the operating period itself for such electrical appliances that require the power at all times.
The switching power supply device itself converts an alternating-current (AC) voltage to a direct-current (DC) voltage by rectifying the AC voltage through a rectifying circuit thereof and by smoothing a resultant undulating voltage through a smoothing circuit thereof. The DC voltage thus obtained is switched on and off by a switching element and fed to an output rectifying smoothing circuit for rectifying and smoothing processes to obtain any given predetermined DC voltage.
In such a switching power supply device as mentioned above, if the smoothing circuit at an input side is a capacitor-input type, there is a problem in which a power factor is reduced, because the input current flows only when a rectified voltage becomes higher than a charged voltage of an input smoothing capacitor and a conduction angle of an input current becomes smaller accordingly. To solve this problem, switching power supply devices equipped with a booster chopper circuit having a power-factor improvement function have been conventionally used.
Also, the Japanese Patent Application Laid-Open No. 2001-95236 discloses a switching power supply device that has a power-factor improvement function by using an output power sensing circuit for outputting a control signal so that the power-factor improvement function of a booster chopper circuit thereof is deactivated when the output power is less than a predetermined amount and that the power-factor improvement function of the booster chopper circuit thereof is activated when the output power is more than the predetermined amount.
These conventional switching power supply devices equipped with the booster chopper circuit having the power-factor improvement function contribute to reducing the power consumption, because a reactive power is reduced by the improved power factor. However, in comparison with a switching power supply device having no power-factor improvement function, these conventional switching power supply devices give rise to a loss of power required for operating the power-factor improvement function of the booster chopper circuit and a power conversion efficiency thereof drops accordingly. The conventional switching power supply devices waste unnecessary power by operating the power-factor improvement function, particularly in a low-power consumption state in which improvement of the power factor is not necessary during such a period as a standby period of the electrical appliances.
The conventional technology disclosed in the Japanese Patent Application Laid-Open No. 2001-95236 is capable of preventing the wasteful power required for the operation of the power-factor improvement function from being consumed by stopping the operation of the power-factor improvement function in the low-power consumption state. To do so, the switching power supply device requires an output power sensing circuit for detecting the low-power consumption state and a power-factor improvement function control circuit for stopping the power-factor improvement function according to a control signal fed from the output power sensing circuit, which, in return, causes the circuitry to become complicated.